Fundamental Galactic Parameters

Despite decades of effort by astronomers, some fundamental parameters
such as the size and rotation rate of the Milky Way are not well known.
One parameter we would like to determine is the distance from
the Sun to the center of the Galaxy (Ro).
All models of the Milky Way and its mass scale with this important
parameter. Current estimates range
from Ro = 5.0 to 8.8 kpc (where 1 kpc = 3.086 x
1019 meters). We will use SIM PlanetQuest to determine this
parameter to a higher level of accuracy, by measuring the parallaxes
of stars that are believed to be very near the center of the Galaxy.

The Local Standard of Rest (LSR) is
defined as the orbit of an object around the center
of the Milky Way at the solar circle (the radius of
the Sun's orbit). The LSR is
a function of how much mass (or gravity) is interior to that
orbit. The LSR is not exactly the motion of the Sun, because
the Sun is not exactly on a circular orbit, but the two are
very close. Click
here for more details.

The Sun and other stars can deviate from the LSR by
interacting with other disk material. Using SIM PlanetQuest, we
would like to find the motion of the Sun relative to
the LSR. This value
(LSR),
which has previously been determined by a number of methods, ranges from
(219 20) (Ro / 8) km/s to
184 8 km/s.

The LSR actually sets the magnitude of the rotation
curve of the Galaxy. The rotation curve of the Galaxy
is measured relative to the motion of the Sun; without
knowing how fast the Sun moves, we don't know the actual
speed of the rotation curve. In fact, we know the rotation
curves of other galaxies much better than we know the rotation
curve of our own. We can determine the rotation curves of
other galaxies using observations of atomic hydrogen gas with radio telescopes. Unfortunately, this method does not work
outside the solar circle!

Our goal in this project will be to provide the first rotation
curve of the Galaxy over a large range of radii using stars
of known distance.

Also, since we don't know the rotation curve of the Milky Way,
we can't relate it to other galaxies, e.g., using the Tully-Fisher
relation. The Tully-Fisher relation states that for most galaxies,
the rate of rotation is proportional to the galaxy's mass or
luminosity.

With SIM PlanetQuest we will be able to measure Ro
as well as the angular velocity of the Sun
(o). From these values,
we will be able to derive LSR.
An accurate determination of these parameters is not only essential for
determining the mass of the Milky Way, but is important for virtually every
problem in Galactic astronomy. Using SIM PlanetQuest, we will be able to determine the
values for Ro and LSR
to an error of 3%, leading to an error of 5% for the mass of the Milky Way.

We will use SIM PlanetQuest to investigate
the motion of the Galactic bar. By obtaining proper motions for
about 10,000 bulge stars, we will be able to constrain the properties
(shape, mass, and pattern speed) and history of the Galactic bar.
This will be important for interpreting the dynamics of the disk,
and for determining if the bar is affecting the outer parts of the
Galaxy.